You are here:
STABILITY: AN INVESTIGATION OF AS(III)/AS(V) STABILITY IN IRON RICH DRINKING WATER MATRICES
Citation:
Gallagher, P A., J T. Creed, C A. Schwegel, A. Heck, AND B. M. Gamble. STABILITY: AN INVESTIGATION OF AS(III)/AS(V) STABILITY IN IRON RICH DRINKING WATER MATRICES. Presented at International Ion Chromatography Symposium, Chicago, IL, September 9-12, 2001.
Impact/Purpose:
To develop an in-field preservation strategy for As(III) and As(V) in iron rich drinking waters which will allow samples to be shipped to the laboratory for arsenic speciation analysis. The overall goal is to improve treatment decision making and in turn minimize exposure to arsenic resulting from the consumption of drinking water.
Description:
Arsenic in drinking water is predominantly inorganic arsenic. The two oxidation states of inorganic arsenic are As(III)(pKa=9.3) and As(V)(pKa2=6.9). The distribution of As(III) and AS(V) in a water is dependent on the redox potential of the water. The actual distribution can vary considerably with a general rule that high percentages of As(III) are ususally associated with ground wqters while high percentages of As(V) are generally associated with surface waters. Current drinking water regulatins do not differentiate between these two oxidation states when setting the maximum contaminate level (MCL) for arsenic. The need to differentiate As(III) and As(V) arises from a treatment/removal perspective within the drinking water regulations. Certain treatment strategies remove As(V) more efficiently than As(III). Therefore, treatment engineers usually require a speciation based analysis before designing a treatment strategy. The accuracy of this analysis is strongly dependent on how well the native distribution of As(III) and As(V) is preserved prior to analysis. Factors which can influence the preservation are dissolved oxygen, iron preciption, temperature, time and biological actvity. The precipitation of iron is an important factor because it can change the distribution and the contration of aqueous arsenic. In addition, there is a geological correlation between arsenic and iron, thus the iron precipitaiton factor is quite common. In an attempt to preserve the native arsenic distribution in iron rich waters we have been investigating the use of EDTA in combination with acidification with acetic acid (HAc). This presentation will focus on the use of EDTA/HAc in a number of well waters from across the US. The stability of the EDTA/HAc treated samples will be investigated over time and compared to controls and non-treated samples. The overall change in the distribution will be reported with a special emphasis on how these changes affect the treatment decision making process.